"Capture" is defined as a cardiac response to a pacemaker stimulation pulse. When a pacemaker stimulation pulse stimulates either the heart atrium or the heart ventricle during an appropriate portion of a cardiac cycle, it is desirable to have the heart respond to the stimulus provided. Every patient has a threshold which is generally defined as a minimum amount of stimulation energy required to effect capture. It is usually desired to achieve capture at the lowest possible energy setting yet provide enough of a safety margin so that should the patient's threshold increase, the output of an implanted pacemaker would be sufficient to maintain capture.
Capture is usually assessed by means of an electrocardiogram (ECG) measured through ECG electrodes placed on the patient's limbs and/or chest. When a patient is connected to a typical ECG monitor and the pacemaker is providing stimulation pulses, the physician monitors the output to assess whether each pacing pulse, which is seen as a spike, is followed by a cardiac response. Ventricular capture is relatively easy to assess in that each ventricular stimulation produces a very large R-wave. Determination of atrial capture in response to an atrial stimulation pulse is a more difficult task. Atrial capture in response to stimulation pulses has been viewed on an electrocardiogram as P-waves following each atrial stimulation by a constant time interval. One prior art embodiment utilized dual sensing electrodes and suggests the heart action is 15 to 20 milliseconds after the stimulus. (See Goldreyer, U.S. Pat. No. 4,365,639.) However, the time delay varies considerably depending on the patent, administered drugs, electrolyte balance, proximity of sensing electrode to stimulating electrode and other factors. Further, it is almost impossible to guarantee that a P-wave will be of a sufficient amplitude to be seen on a standard ECG scan. In order to verify atrial capture in patients with intact cardiac conduction, the physician must pace atrially and observe ventricular response to the paced atrial rate. However in patients with heart block, the physician may not be able to determine atrial capture because of the lack of conduction from the atrium to the ventricle, thus preventing the ventricle from responding to atrial stimulation pulses. In such cases the physician may have to rely on fluoroscope evaluation of cardiac wall motion in response to the atrial stimulation to ascertain atrial or P-wave capture.
Another method for determining atrial capture is to transmit the signal appearing on the atrial stimulation electrode to an external viewing device. Some of the newer pacemakers have the capability to transmit electrogram (EGM) signals appearing at either the atrial electrode or the ventricular electrode in real time to an external monitoring device for real-time evaluation by a physician. (see, for example, U.S. Pat. No. 4,232,679 to Schulman.) However, due to the large magnitude of a stimulation pulse with respect to the P-wave signal, and the closeness in time between the stimulation pulse and the occurrence of the P-wave, the atrial sensing amplifiers of conventional pacemakers saturate in the presence of a stimulation pulse and mask the P-wave. Thus as a practical matter, utilization of EGM signals appearing at the stimulation electrode is not effective for determining if P-wave capture has occurred. One feature of the present invention solves this problem by providing an apparatus for determining P-wave capture through use of a conventional implantable bipolar atrial electrode without having to utilize the stimulation electrode for P-wave detection.